1. Field of the Invention
The present invention relates to damping devices for elongated waveguides in magnetostrictive displacement or distance measuring transducers, and more particularly to modularly constructed magnetostrictive -transducers having damping, including for displacement or distance measuring.
2. Description of the Art
Magnetostrictive transducers having elongated waveguides that carry torsional strain waves induced in the waveguide when current pulses are applied along the waveguide through a magnetic field are well known in the art. A typical linear distance measuring device using a movable magnet that interacts with the waveguide when current pulses are provided along the waveguide is shown in U.S. Pat. No. 3,898,555. Damping devices for waveguide are illustrated in this patent, to prevent reflected sonic strain waves at both the remote end of the waveguide and the mounted end of the waveguide. These devices generally are soft rubber pads that are clamped about the waveguide to absorb sonic strain wave pulse energy to minimize reflections of the generated pulse and reduce interference of the reflections with the signals to be sensed.
The damping devices and the arrangement for anchoring the waveguide at a remote end may take up a substantial length at the end remote from the signal source of a sonic waveguide for prior art of this sort, as discussed in U.S. Pat. No. 3,898,555. Where liquid levels, for example, are being sensed by the transducers, it is desirable to have the waveguide operable and active as close to the bottom of the tank as possible, thereby minimizing the length of the waveguide support at its remote end from the signal source, including the length of the separate clamping device at such end, and at the mounted end of the waveguide, where the signal source is mounted.
It is also known in the art to use silicone rubber dampers of two different durometers and/or different loading pressure against the waveguide. Lower pressure and lower durometer silicone rubber has been utilized to minimize front end reflection (input end) while higher durometer silicone rubber in conjunction with greater clamping pressures has been utilized to provide damping at the remote or termination end. This use of silicone rubber was believed to be a compromise as a damping medium because of its high resilience, which leads to the need for long damping sections. Silicone rubber does have good stability over a wide temperature range which is an important benefit for damping materials.
In addition, in the prior art, the mass density of the damping material may be quite important to provide a mechanical impedance such that the sonic strain wave energy can be transferred into the damping device and dissipated. The coupling of the waveguide to the damping device must also be effective. The dissipation of the sonic strain wave energy by the damping medium has been thought in the prior art to provide the damping.
It is also known in the art to use gum type damping media because of the ability to attenuate or damp vibration but such materials harden at temperatures which are near the freezing point of water and become extremely soft at temperatures well below 200 degrees Fahrenheit. The same is true for epoxy or urethane elastomers, and such large changes in characteristics change the "front" end reflection and the "extreme" end reflection characteristics drastically with temperature.
The need for an effective damp material is especially evident when the transducer uses what is known as recirculation mode sensing. In the recirculation mode, each time the sensor receives a sonic strain wave signal, a new current pulse is sent, and this leads to a high frequency of sonic strain wave pulses and a build up of noise as a result of reflections. If effective damping is not provided, "noise" build-up reduces the usefulness of the sensing technique, especially since the sonic strain wave signal is known in the prior art to be of low amplitude. Thus, in the prior art, it was ideal for the damp material to be capable of being kept short, along with the end mounting structure for the waveguide, to have good coupling to the waveguide itself, and to have the ability to dissipate energy, the total of which is not well achieved in the prior art. For other approaches raising signal strength, see U.S. Pat. No. 4,952,873.
An alternate methodology for damping is set out in U.S. Pat. No. 4,958,332. This patent teaches an improved damping method. The damping device comprises a highly viscous, flowable material that adheres to and couples to the waveguide, and which can have mass density changing additions, such as metallic powder, to vary the mass density along its length. The damping material is held against the waveguide with a suitable housing which can be loaded against the waveguide with pressure as selected. While such a method is effective, it is difficult to produce.
Also in the prior art, two half pieces (flat sheets) of rubber have been used to enclose a waveguide with a metal clamp to retain them around the waveguide and apply pressure to the waveguide at the input side, but this is fairly expensive.
For general background information, see "Ultrasonic Level, Temperature and Density Sensor" by S. C. Rogers and G. N. Miller, IEEE Transactions on Nuclear Science, Vol. NS-29, No. 1, February 1982.
It is an object of the present invention to produce a waveguide damping mechanism which is easy to produce and assemble.